This invention relates to an electrolyte capable of anodizing aluminum and which consists essentially of a solution of an amino acid having a pH of 5.5 to 8.5. This invention relates also to an electrolyte which can be used to anodize aluminum to produce a low voltage (0-125 V) barrier layer dielectric oxide on the aluminum surface or as a fill electrolyte in low voltage (0-63 V) aluminum electrolytic capacitors. More specifically, it relates to a solution of a 2-amino acid, preferably a dicarboxylic acid, in water or an organic electrolytic capacitor solvent.
Salts of organic acids have been used as solutes in electrolytes in the aluminum electrolytic capacitor industry. Aqueous solutions of acid salts, e.g., citrates, tartrates, adipates, have been used as anodization or formation electrolytes while these and others have been used in non-aqueous operating or fill electrolytes in aluminum electrolytic capacitors.
Various problems have been encountered when the aqueous solutions of organic salts have been utilized as anodization electrolytes. In aqueous solutions of salts of .alpha.-hydroxycarboxylic acids (e.g. tartrates, citrates etc.), the current efficiency of oxide formation is very low. In aqueous adipate solutions, the anodic oxide which is formed on aluminum is very susceptible to hydration degradation during its exposure to various working electrolytes in capacitors. In addition, while these electrolytes appear to be useful in forming a higher capacitance dielectric film on aluminum, this dielectric film is often unrelaxed, and of lower capacitance than would be truly desirable.
Several methods have been utilized to overcome the problems of low current efficiency of oxide formation, easy degradation of anodic oxide by hydration, and unrelaxed oxide formation. The problem of poor anodizing efficiency has been attacked in the past by treatment of the aluminum foil surface with boiling water or high heat (600.degree. C.) to introduce respectively a protective hydrous or thermal oxide film prior to anodization. The problem of hydration degradation has been dealt with by performing anodization in mixed solutes, one of which imparts hydration resistance, or in stages in which an electrolyte imparting hydration resistance is used in at least one of the stages. Unrelaxed oxide films have been relaxed by boiling water treatments. Higher capacitance has been achieved also (apart from changing solutes) by introducing a hydrous or thermal oxide film prior to anodization.
The development of an electrolyte capable of directly forming a stable, high capacitance oxide on aluminum is therefore desirable.